A vehicle may decelerate from time to time when a driver releases an accelerator pedal. The vehicle may decelerate at a constant rate (or at least consistently) until the driver requests torque for acceleration or until vehicle speed is zero or within a threshold speed of zero. By decelerating at a constant rate, a driver may have a sense of when the vehicle will reach zero speed and is an expected vehicle response characteristic. However, if the constant rate of deceleration is large, the vehicle may suffer from lack of fuel economy since the vehicle is losing its kinetic energy to powertrain braking losses. Conversely, if the constant rate of deceleration is small, the driver may have a feeling that the vehicle is maintaining it present speed. Therefore, it may be desirable to provide a rate of vehicle acceleration that provides good fuel economy while reducing the perception that the vehicle is not decelerating.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for operating a vehicle, comprising: decelerating a vehicle via freewheeling a driveline in response to a demand torque less than a threshold, vehicle speed greater than a threshold, and traffic conditions that do not indicate expected driver braking within a predetermined vehicle travel distance.
By freewheeling a driveline during conditions when demand torque is less than a threshold, vehicle speed is greater than a threshold, and traffic conditions that do not indicated expected driver braking within a predetermined vehicle travel distance, a vehicle's coasting distance may be increased to improve vehicle fuel economy. Further, driveline braking may be increased via increasing an air conditioner compressor load on an engine that is rotating but not combusting air and fuel in response to expected driver braking so that a vehicle's kinetic energy may be converted into useful work or stored energy. For example, a transmission may be shifted into neutral when small decelerations are desired to increase the vehicle's coasting range or distance. The transmission may be shifted into gear with the torque converter locked while the engine is unfueled and spinning to provide a higher level of driveline braking for moderate to higher levels of desired deceleration. Activating an air conditioner compressor and increasing alternator load while the transmission is shifted into gear while the torque converter is locked and while the engine is rotating unfueled may provide even greater levels of driveline braking during conditions when high braking levels are desired or expected. In these ways, driveline braking may be adjusted to provide more useful benefit than simply applying a fixed driveline braking amount during conditions of low torque demand.
The present description may provide several advantages. For example, the approach may extend a vehicle's coasting distance during conditions when very little driveline braking is desired. Further, the approach may provide a desired amount of driveline braking power to decelerate a vehicle during conditions where an adjustable amount of driveline braking may be desirable.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.